A wire-bonding method has conventionally been used as a method for mounting an electronic component element such as a semiconductor element or a solid-state image pickup device on a substrate such as a circuit substrate or a package carrier. The wire-bonding method is a method for obtaining an electrical connection by bonding both ends of an extremely thin wire to respective electrodes of an electronic component element and a substrate.
However, a flip-chip bonding method, which has higher production efficiency, has recently been used. The flip-chip bonding method is a method for bonding an electrode of an electronic component element to an electrode (a bonding terminal) of a substrate via a bump (a protrusion electrode) which is an electrically conductive connecting member. The flip-chip bonding method makes it possible to collectively bond, via bumps, a plurality of parts to be bonded, and thus has an advantage of having higher production efficiency as compared with the wire-bonding method in which parts to be bonded are basically bonded one by one in sequence via an extremely thin wire. According to the flip-chip bonding method, electrodes which are bonding terminals of a substrate do not need to be provided close to an electronic component element. This makes it possible to greatly increase the number of bonding terminals, reduce an area for mounting the electronic component element, and shorten a wire length of a circuit. Accordingly, the flip-chip bonding method is suitable for, for example, high-density or high-speed mounting of electronic component elements.
Specific examples of the flip-chip bonding method encompass (i) a method for bonding a bump to an electrode of a substrate via an intermediate material such as an electric conductive paste and (ii) a method for directly bonding a bump to an electrode of a substrate by thermocompression bonding or thermocompression bonding with which an ultrasonic wave is used in combination. The method (ii) has an advantage of reducing the number of steps without the need to provide an intermediate material, and a further advantage of reducing time required for the bonding. Therefore, the thermocompression bonding (an ultrasonic flip-chip bonding method), with which an ultrasonic wave is used in combination, has recently been frequently used as the flip-chip bonding method.
According to the ultrasonic flip-chip bonding method, a bump is typically bonded to an electrode of a substrate by applying an ultrasonic vibration to the bump while a constant load is being applied to the bump.
The ultrasonic flip-chip bonding method has a problem of low reliability of an electric connection due to an insufficient bonding strength. The problem can be solved by a commonly known method for bonding a bump to an electrode of a substrate by gradually increasing a load and an output of an ultrasonic wave which are applied to the bump. According to the above bonding method, the load and the ultrasonic wave simultaneously start to be applied to the bump, and thus the ultrasonic wave is applied to the bump in a state in which a tip of the bump has not been sufficiently crushed. That is, the ultrasonic wave is applied in a state in which the bump is hardly bonded to the electrode of the substrate. This causes an electronic component element and the substrate to slip due to an ultrasonic vibration applied by an ultrasonic vibrator. Accordingly, the electronic component element is moved, so that the electronic component element and the substrate are relatively positionally displaced. In a case where the load and the output of the ultrasonic wave which are applied to the bump are both increased in such a state as described above, there occurs a problem such that the bump is bonded to the electrode of the substrate while respective parts of contact between the bump and the electrode of the substrate are positionally displaced.
In view of the problem, Patent Literatures 1 and 2 each disclose a devised method for carrying out bonding excellent in bonding strength by bringing bumps into uniform contact with respective electrodes of a substrate. FIG. 6 illustrates a transition of an amount of a load to be applied and a transition of a state of output of an ultrasonic wave in a bonding method disclosed in Patent Literature 1. (a) of FIG. 6 illustrates the transition of the amount of the load to be applied. (b) of FIG. 6 illustrates the transition of the state of output of the ultrasonic wave. As illustrated in FIG. 6, the bonding method disclosed in Patent Literature 1 includes: (i) a first step of bringing a protrusion electrode of an electronic component element into contact with a pad electrode while increasingly applying a load to the protrusion electrode; (ii) a second step of fusion-bonding the protrusion electrode to the pad electrode by application of an ultrasonic vibration while increasingly applying a load to the protrusion electrode; and (iii) a third step of fusion-bonding the protrusion electrode to the pad electrode by application of an ultrasonic vibration while applying a constant load to the protrusion electrode. According to the disclosure in Patent Literature 1, even in a case where a difference in height occurs in the substrate or the bumps vary in height, the above bonding method makes it possible to carry out bonding excellent in bonding strength by bringing bumps into uniform contact with respective electrodes of a substrate.
Patent Literature 2 discloses a bonding method including: (i) a first step of, by lowering a bonding tool at a given speed while causing the bonding tool to apply an ultrasonic wave to a bump, controlling a speed at which the bump is crushed; and (ii) a second step of, after the first step, by applying a given pressing load to the bonding tool while causing the bonding tool to apply an ultrasonic wave to the bump, bonding the bump to a surface to be bonded. According to the disclosure in Patent Literature 2, the above bonding method makes it possible to gradually crush the bump and sufficiently transmit an ultrasonic vibration to a bonding surface, so that a higher bonding strength can be obtained.